Question

Why does the magma inside the earth and other planets have not cooled down since their birth millions and billion years ago?

Answer 1

Answer:

The answer lies in the fact that the earth is extremely huge, some other planets many times larger than the Earth. The Magma at the center of the Earth has no outlets except tiny bits in the form of volcanic Lava. It will keep burning and so will the Sun. We have no means of measuring how long. It would be like counting the grains of sand in the Sahara Desert.